This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) deposited on a surface to have a columnar microstructure, wherein the TBC overlying at least certain portions of the surface has an interior region that is denser than an exterior region overlying the interior region to improve the impact resistance of the TBC.
Components within the hot gas path of gas turbine engines are often protected by TBC's that are typically formed of ceramic materials deposited by plasma spraying, flame spraying, and physical vapor deposition (PVD) techniques. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can also be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
Various ceramic materials have been proposed as TBC's, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2) to yield a tetragonal microstructure that resists phase changes. Though various other stabilizers have been proposed for zirconia, yttria-stabilized zirconia (YSZ) is often preferred due at least in part to its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying, and PVD techniques. Nonetheless, considerable effort has been made to formulate ceramic materials with reduced thermal conductivity, improved resistance to spallation and sintering, and other properties and characteristics that detrimentally affect the thermal insulating capability of a TBC.
In addition to low thermal conductivity and spallation resistance, TBC's on gas turbine engine components are required to withstand damage from erosion and impact by particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The damage can be in the form of erosive wear (generally from smaller particles, lower particle velocities, and/or lower impingement angles) and impact spallation (generally from larger particles, greater particle velocities, and/or greater impingement angles). Commonly-assigned U.S. Pat. No. 5,981,088 to Bruce et al. teaches that YSZ containing less than six weight percent yttria exhibits improved impact resistance. In addition, commonly-assigned U.S. Pat. No. 6,352,788 to Bruce and U.S. Pat. No. 7,060,365 to Bruce teach that small additions of oxides such as magnesia, hafnia, lanthana, neodymia, and/or tantala can improve the impact and erosion resistance of zirconia partially stabilized by about four weight percent yttria (4% YSZ). Aside from compositional approaches, improvements in erosion and impact resistance have been achieved by forming the outer region of a PVD TBC to be denser than an underlying interior region of the TBC, as taught in commonly-assigned U.S. Pat. No. 5,683,825 to Bruce et al. and commonly-assigned U.S. Pat. No. 6,982,126 to Darolia et al.
Notwithstanding the above-noted advancements, it would be desirable if TBC's were available that exhibited further improvements in resistance to particle damage, and particularly impact damage.